With the increase in the amount of transmission information, optical interconnect lines in addition to electrical interconnect lines have been used in recent electronic devices and the like. As an example, an opto-electric hybrid board has been proposed, as shown in FIG. 4 (see PTL 1, for example). This opto-electric hybrid board includes: an electric circuit board E1 including an insulative layer 51, and electrical interconnect lines 52 formed on the front surface of the insulative layer 51; an optical waveguide W1 [including a first cladding layer 56, a core (optical interconnect line) 57 and a second cladding layer 58] stacked on the back surface (surface opposite from the surface with the electrical interconnect lines 52 formed thereon) of the insulative layer 51 of the electric circuit board E1; and a light-emitting element 11 and a light-receiving element 12 which are mounted on portions of the surface with the electrical interconnect lines 52 formed thereon, the portions being corresponding to opposite end portions of the optical waveguide W1. In this opto-electric hybrid board, the opposite end portions of the optical waveguide W1 are formed into inclined surfaces inclined at 45 degrees with respect to the longitudinal direction (direction in which light propagates) of the core 57. Portions of the core 57 positioned at the inclined surfaces function as light reflecting surfaces 57a and 57b. Portions of the insulative layer 51 corresponding to the light-emitting element 11 and the light-receiving element 12 have respective through holes 55 for an optical path. The through holes 55 allow light L to propagate therethrough between the light-emitting element 11 and the light reflecting surface 57a provided in a first end portion and between the light-receiving element 12 and the light reflecting surface 57b provided in a second end portion.
The propagation of the light L in the aforementioned opto-electric hybrid board is performed in a manner to be described below. First, the light L is emitted from the light-emitting element 11 toward the light reflecting surface 57a in the first end portion. The light L passes through one of the through holes 55 for an optical path formed in the insulative layer 51, and then passes through the first cladding layer 56 in the first end portion (left-hand end portion as seen in FIG. 4) of the optical waveguide W1. Then, the light L is reflected from the light reflecting surface 57a in the first end portion of the optical waveguide W1 (the optical path is changed by 90 degrees), and travels through the interior of the core 57 in the longitudinal direction thereof. Then, the light L propagated in the core 57 is reflected from the light reflecting surface 57b in the second end portion (right-hand end portion as seen in FIG. 4) of the core 57 (the optical path is changed by 90 degrees), and travels toward the light-receiving element 12. Subsequently, the light L passes through and exits from the first cladding layer 56 in the second end portion. Then, the light L passes through the other of the through holes 55 for an optical path, and is received by the light-receiving element 12.
Unfortunately, the light L emitted from the light-emitting element 11 and the light L reflected from the light reflecting surface 57b in the second end portion are diffused, as shown in FIG. 4. For this reason, the light L is effectively propagated in small quantity. This results in high propagation losses of the light L.
To solve such a problem, there has been proposed a structure in which the light reflecting surface 57b is made in the form of a concave surface 69a and in which gold is evaporated onto the outside surface thereof to forma reflecting mirror 69, as shown in FIG. 5 (see PTL 2, for example). In this structure, the light L propagated in a core 67 is reflected from the concave surface 69a of the reflecting mirror 69 in an end portion of the core 67, so that the light L is collected in a light-receiving portion 12a of the light-receiving element 12. Thus, an attempt has been made to reduce the propagation losses of the light L. In FIG. 5, the reference numerals 66 and 68 designate cladding layers.